How to be a Mathematical Clairvoyant

Requirements: Spooky music and spooky look.

Tell your students you will beat them adding up 5 x 5 digit numbers in your head when they are using calculators.

Method:

1. Ask a student to write down 2 x 5-digit numbers on the board.

2. You rapidly write a 5-digit number underneath.

3. Ask another student to write another 5-digit number.

4. You write another 5-digit number quickly.

5. You have 5 by 5 digit numbers. Say ‘Go’. You instantly write down the answer.

Stand back.

This is how it works:

N1 = 97413

N2 = 28619

N3 = 71380 (Each digit in N3 that you write down must add up to 9 with digits in No. above)

N4 = 64231

N5 = 35768 (Once again each digit in N5 must add up to 9 with digits above)

Now you will instantly write down the sum of these five numbers as

297411

Da! DA!

The trick is to subtract 2 from N1 and put it in front:

N1 = 97413

N1 -2 = 97411

Sum of 5 numbers = 297411

This is why it works:

Hint: When you get your students to add up the five 5-digit numbers on a calculator you will beat them, but they will also get many different answers as a number of students will key incorrect numbers.